Tetrachloroethylene also called perchlorethylene or PCE) is a solvent commonly used for decreasing operation and as a dry cleaning agent. It is also a common contaminant in public drinking water supplies. Both epidemiological and toxicological data implicate PCE as a carcinogen. Our prior research suggests that PCE-contaminated drinking water may be associated with an increased risk of breast cancer. We conducted a population-based case-control study including 258 breast cancer cases diagnosed from 1983 through 1986. Subjects were exposed to PCE when it leached from the plastic lining of drinking water distribution pipes from the late 1960s through the 1970s. The adjusted relative risks of breast cancer among "highly' exposed subjects ranged from 1.3 when latency was ignored to 2.4 when 9 years of latency was applied. However, the increased risks were not statistically significant because th proportion of exposed subjects was small, particularly when long latent periods were hypothesized. Thus, a larger case-control study of more recently diagnosed cases is needed to estimate precisely the risk for different latency assumptions. The proposed epidemiologic study will test the hypothesis that PCE found in the distribution systems of public drinking water supplies in the Cape Cod region of Massachusetts is associated with breast cancer. This will be accomplished by conducting a population-based case-control study including 1,170 cases diagnosed from 1987 through 1995 and 1,170 controls and: (1) Estimating PCE exposure in breast cancer cases and controls using a well- developed exposure model; (2) Determining the relationship between breast cancer risk and estimated PCE exposure while controlling for confounding variables; and (3) Evaluating the impact of residential water use for drinking, cooking, and bathing on the relationship between breast cancer risk and estimated PCE exposure.